1. Field of the Invention
The invention relates generally to the manufacture of polymer films. More specifically, the invention relates to polymer films that have been seamed and a method of seaming sheets of polymeric material together with diffusion welding.
2. Background Art
Polymer films are frequently produced in a rolled form. Such rolls are often seamed together to combine multiple rolls, to make a longer roll, or to make a larger polymeric sheet structure. The films can be thin, such as 2 mils thick or less, but the films can also be thicker. Thin sheets are often sensitive to mechanical tears and contamination. Some materials are not as strong as others, so thin sheets are even more prone to mechanical tears, rips, or other damages.
Thermoset plastics are polymeric materials that irreversibly cure. The cure may result from heat, irradiation, or a reaction. The reaction can be between two different compounds, such as with an epoxy. Some polymers are formed with two different types of monomers, such as a polyimide. Most polyimides are considered an AA-BB type polymer because two different classes of monomers are used to produce the polyimide polymer. One class of monomer is called an acid monomer, and is usually in the form of a dianhydride. The other type of monomer is usually a diamine, or a diamino monomer.
Thermoset plastics often have no melting point. The thermoset plastic may decompose before melting, or the thermoset plastic may begin decomposing as it approaches its melting point. Thermoset plastics will generally have a glass transition temperature, where the plastic becomes less brittle and softer above the glass transition temperature. Because thermoset plastics do not have a true melting point, or they begin to decompose as they approach their melting point, melt welding is not effective in joining separate pieces. Other techniques are needed to join sheets of thermoset plastic together. Thermoplastic materials, on the other hand, can be melted without significant decomposition. Sheets of thermoplastic materials can be overlapped and melted together, which is referred to as melt welding. Thermoplastic materials do have a glass transition temperature which is lower than its melting point. Polymeric materials can be thermoplastic or thermoset.
Polyimides are one type of thermoset plastic with many desirable characteristics. Other thermoset plastics include polybenzoxazoles and epoxies. There are also examples of polyimides, polybenzoxazoles, and epoxies which are thermoplastic. Polyimide sheets can be used for space applications where weight is an important factor. Thinner sheets weigh less, but still need to be strong enough to function. Also, the temperature in space can change significantly and rapidly. If two different materials are joined together, and the different materials have different coefficients of thermal expansion (CTEs), the rapid and large temperature changes can cause the materials to separate. An adhesive and a polymer will often have different CTEs.
Thin sheets of polyimides are commonly used on satellites and other space flight equipment. Thin sheets are also referred to as films in this disclosure. They are typically thin polymer sheets which may have an applied reflective metallized evaporative coating. These sheets are used as solar shields, solar concentrators, solar sails, etc. A reflective membrane may be used to protect orbital structures and equipment such as satellites from direct exposure to solar radiant flux. Alternatively, a reflective membrane may be used to concentrate solar energy on equipment such as a solar panel that powers a satellite. Other coatings can also be used for various applications.
Prior art techniques of seaming include bonding the materials with an adhesive or alternatively employing a mechanical fastener. However, the application of the adhesive seaming chemicals is challenging and prone to contamination. Additionally, commonly used adhesives, such as acrylic-based or silicone-based, lose mechanical strength and increase in stiffness at temperatures below their glass transition temperature (Tg), such as temperatures encountered in space. Differences in the CTE between the polymer and the adhesive can also lead to premature seam failure. A mechanical fastener, such as stitching with thread, is prone to tearing especially if single layers of materials are seamed together. Consequently, a method for effectively seaming thin polymer sheets is needed.